1. Field of the Invention
The present invention relates to a gas sensor element and a gas sensor which detects a concentration of a detecting gas.
2. Description of the Related Art
A sensor in which a substantially cylindrical gas sensor element extending in an axial direction and having a closed front end is inserted into and held in a tubular main metal fitting has been known as a gas sensor which detects an oxygen concentration in an exhaust gas of a vehicle or the like (see JP 2012-37445A). This gas sensor element has a tubular solid electrolyte body (hereinafter, also referred to as an element main body), and an inside electrode and an outside electrode which are formed on inner and outer surfaces of the solid electrolyte body, respectively. The outer surface of the solid electrolyte body is covered with a porous protective layer for protecting the outside electrode from poisoning or the like (see JP-H11-72460A).
A technique has been developed in which in a gas sensor element provided with a porous protective layer on a lamination type (plate type) detection element, the porous protective layer includes three regions. Further, the inside region has a higher porosity than an outside region (see JP 2012-189579A).
In the porous protective layer of JP 2012-189579A, the porosity of the inside region is higher than the porosity of the outside region. Thus, a heat insulating effect of the inside region is increased, and loss of the heat of the detection element to the porous protective layer can be suppressed. On the other hand, since the porosity of the outside region is lower than the porosity of the inside region, a poisonous substance adhering to the porous protective layer can be effectively caught in the outside region. Further, water droplets adhering to the porous protective layer have difficulty penetrating the inside region, but can be allowed to effectively penetrate into the outside region. As a result, adhesion of the water droplets to the detection element can be suppressed.
In recent years, the porous protective layer of JP 2012-189579A has been considered for application to a tubular gas sensor element. However, this porous protective layer is easily peeled from a solid electrolyte body when applied to a tubular gas sensor element. The detection element of a lamination type (plate type) gas sensor element has a relatively lower strength than the solid electrolyte body of the tubular gas sensor element. Accordingly, when a gas sensor is formed by assembling the lamination type (plate type) gas sensor element to a main metal fitting and a vibration is imparted to the gas sensor, the detection element is bent together with the porous protective layer. Thus, the stress added only to the porous protective layer can be released.
On the other hand, the solid electrolyte body of the tubular gas sensor element has a relatively high strength. Accordingly, when a gas sensor is formed, the solid electrolyte body is not bent even when a vibration is imparted to the gas sensor. Therefore, the stress is added only to the porous protective layer, and thus the porous protective layer is easily peeled from the solid electrolyte body. Particularly, when a porous protective layer in which an inside region has a higher porosity than an outside region as in JP 2012-189579A is applied to the tubular gas sensor element, the porous protective layer is easily peeled at an interface with the solid electrolyte body.